System and method for increased signal-to-noise ratio in multi spin-echo pulse imaging

ABSTRACT

The present disclosure provides a system for and a method of obtaining a magnetic resonance image by performing magnetic resonance imaging (MRI) at multiple slices simultaneously. The method comprises generating a multiband pulse sequence for spin-echo imaging, the pulse sequence comprising a multiband excitation pulse and at least one multiband refocusing pulse, wherein the multiband excitation pulse simultaneously excites multiple bands, wherein the at least one multiband refocusing pulse simultaneously refocuses the multiple bands, and wherein the phases of the bands excited by the multiband excitation pulse and the phases of the bands refocused by the at least one multiband refocusing pulse are set according to a single row of an orthogonal encoding matrix. The multiband excitation pulse and the at least one multiband refocusing pulse collectively form a multiband pulse pair.

FIELD

The present disclosure relates generally to magnetic resonance imaging(MRI) and more specifically, to improving the signal to noise ratio inimages acquired using lower magnetic field MRI systems.

BACKGROUND

The signal to noise ratio (SNR) is used in MRI to describe imagequality, the relative contributions to a detected signal of the truesignal and random signals, or background noise.

A voxel with a larger volume contains more signal, and therefore has ahigher SNR. Longer sampling time tends to reduce the noise, andtherefore increases the SNR. In addition, the MRI hardware contributesto the SNR through the main magnetic field strength, the receive coilsensitivity and volume, and the receive chain noise performancecharacteristics. Finally, the tissue itself can contribute to the signalas determined by its relaxation and other characteristics that affectthe specific pulse sequence being used.

In order to improve the SNR, typically, either the voxel size has toincrease or the sampling time has to increase.

MRI systems which use lower magnetic fields (for example, 0.5 T) tend toresult in a lower sample magnetization than in conventional 1.5 T and3.0 T MRI systems. This also tends to result in a lower SNR in magneticresonance (MR) images, if all other factors are held equal.

A commonly used pulse sequence in MRI is a spin echo sequence. It has atleast two RF pulses, typically a 90° pulse (often called the excitationpulse) and a 180° refocusing pulse that generate the spin echo.Multi-spin echo sequences, or echo-train sequences, are similar.However, they apply multiple 180° refocusing pulses to produce multipleechoes following a single excitation pulse. A refocusing pulse isrequired for every echo produced.

In general, magnetic resonance images are produced over an imagingvolume by selectively exciting and obtaining signals from slices of theimaging volume, using a combination of gradient fields and ‘spatiallyselective’ RF pulses. In a spin echo sequence, the repetition time, TR,is the time between successive excitation pulses for a given slice. Theecho time, TE, is the time from the excitation pulse to the echomaximum. Each slice is excited and repeatedly refocused by a train of180-degree refocusing pulses, with data sampling following each of therefocusing pulses. As such, multi spin-echo sequences may be used toefficiently acquire T2-weighted images.

Multiband encoding involves the excitation of multiple slicessimultaneously within one TR time period, and the summed signal fromthis group of slices is typically sampled following each refocusingpulse. A pulse that is configured to perform this multiband encoding maybe referred to as a multiband pulse.

Simultaneous multislice imaging using multiband RF pulses have been usedin the past to excite multiple frequency bands of magnetization, withthe spatial profile of multiple receiver coils used to separate thesignal from each respective frequency band from the multi-band signal.However, this method is mainly used to enable more slice coverage in thesame imaging time, a form of acceleration and is not used to increaseSNR.

SUMMARY

According to an example aspect, the present disclosure provides amagnetic resonance imaging (MRI) system configured to perform amultiband pulse sequence to acquire a magnetic resonance image atmultiple slices simultaneously, the MRI system comprising: a processorconfigured to transmit a multiband pulse pair. The pulse pair comprisesa multiband excitation pulse for simultaneously exciting multiple bands,and a multiband refocusing pulse for simultaneously refocusing themultiple bands, wherein the phases of each band in the multibandexcitation pulse and the multiband refocusing pulse are set according toa row of an orthogonal encoding matrix. The MRI system furthercomprising a transmit coil coupled to the processor for generating amultiband pulse sequence comprising the multiband excitation pulse andat least one multiband refocusing pulse during spin-echo ormulti-spin-echo imaging.

According to another example aspect, the present disclosure provides amethod of obtaining a magnetic resonance image by performing magneticresonance imaging (MRI) at multiple slices simultaneously. The methodcomprises generating a multiband pulse sequence for spin-echo imaging,the pulse sequence comprising a multiband excitation pulse and at leastone multiband refocusing pulse, wherein the multiband excitation pulsesimultaneously excites multiple bands, wherein the at least onemultiband refocusing pulse simultaneously refocuses the multiple bands,and wherein the phases of the bands excited by the multiband excitationpulse and the phases of the bands refocused by the at least onemultiband refocusing pulse are set according to a single row of anorthogonal encoding matrix, the multiband excitation pulse and the atleast one multiband refocusing pulse collectively forming a multibandpulse pair.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the present disclosure are provided in thefollowing description. Such description makes reference to the annexeddrawings wherein:

FIG. 1 is a block diagram of a magnetic resonance imaging (MRI) systemin accordance with an example embodiment;

FIG. 2 shows an embodiment of a multiband excitation and refocusingpulse sequence that may be generated using the system of FIG. 1;

FIG. 3 is a flow chart showing an example method for performing MRI atmultiple slice locations simultaneously; and

FIG. 4 shows example multiband excitation and multiband refocusingpulses for encoding four slices simultaneously.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Traditional magnetic resonance imaging (MRI) systems represent animaging modality which is primarily used to construct pictures ofmagnetic resonance (MR) signals from protons such as hydrogen atoms inan object. In medical MRI, typical signals of interest are MR signalsfrom water and fat, the major hydrogen containing components of tissues.

Referring to FIG. 1, a block diagram of a MRI system, in accordance withan example implementation, is shown at 100. The example implementationof MRI system indicated at 100 is for illustrative purposes only, andvariations including additional, fewer and/or varied components arepossible.

As shown in FIG. 1, the illustrative MRI system 100 comprises a dataprocessing system 105. The data processing system 105 can generallyinclude one or more output devices such as a display, one or more inputdevices such as a keyboard and a mouse as well as one or more processorsconnected to a memory having volatile and persistent components. Thedata processing system 105 may further comprise one or more interfacesadapted for communication and data exchange with the hardware componentsof MRI system 100 used for performing a scan.

Continuing with FIG. 1, example MRI system 100 also includes a mainfield magnet 110. The main field magnet 110 may be implemented as apermanent, superconducting or a resistive magnet, for example. Othermagnet types, including hybrid magnets suitable for use in MRI system100 are contemplated. Main field magnet 110 is operable to produce asubstantially uniform main magnetic field having strength B0 and adirection along an axis. The main magnetic field is used to create animaging volume within which desired atomic nuclei, such as the protonsin hydrogen within water and fat, of an object are magnetically alignedin preparation for a scan. In some implementations, as in this exampleimplementation, a main field control unit 115 in communication with dataprocessing system 105 may be used for controlling the operation of mainfield magnet 110.

MRI system 100 further includes gradient coils 120 used for encodingspatial information in the main magnetic field along, for example, threeperpendicular gradient axes. The size and configuration of the gradientcoils 120 may be such that they produce a controlled and uniform lineargradient. For example, three paired orthogonal current-carrying primarycoils located within the main field magnet 110 may be designed toproduce desired linear-gradient magnetic fields.

In some implementations, gradient coils 120 may be shielded and includean outer layer of shield coils which can produce a counter magneticfield to counter the gradient magnetic field produced by the primarygradient coils forming a primary-shield coils pair. In such a coil pairthe “primary” coils can be responsible for creating the gradient fieldand the “shield” coils can be responsible for reducing the stray fieldof the primary coil outside a certain volume such as an imaging volume.The primary and shield coils of the gradient coils 120 may be connectedin series.

It is also possible to have more than two layers of coils for any givengradient axis that together form a shielded gradient coil. Shieldedgradient coils 120 may reduce eddy currents and other interference whichcan cause artefacts in the scanned images. Since eddy currents mainlyflow in conducting components of the MRI system 100 that are caused bymagnetic fields outside of the imaging volume (fringe fields), reducingthe fringe fields produced by gradient coils 120 may reduceinterference. Accordingly, the shapes and sizes, conductor wire patternsand sizes, and current amplitudes and patterns of the primary-shieldcoils pair can be selected so that the net magnetic field outside thegradient coils 120 is as close to zero as possible. For cylindricalmagnets, for example, the two coils may be arranged in the form ofconcentric cylinders whereas for vertical field magnets, the two coilsmay be arranged in coaxial disks.

The conductive components of the gradient coils 120, whether shielded orunshielded and including the primary and optional shield coil(s), mayinclude an electrical conductor (for example copper, aluminum, etc.).The internal electrical connections can be such that when a voltagedifference is applied to the terminals of the gradient coils 120,electric current can flow in the desired path. The conductive componentsfor the three gradient axes for both the primary gradient coils and theoptionally gradient shield coils may be insulated by physical separationand/or a non-conductive barrier.

The magnetic fields produced by the gradient coils 120, in combinationand/or sequentially, can be superimposed on the main magnetic field suchthat selective spatial excitation of objects within the imaging volumeoccurs. In addition to allowing spatial excitation, the gradient coils120 may attach spatially specific frequency and phase information to theatomic nuclei placed within the imaging volume, allowing the resultantMR signal to be reconstructed into a useful image. A gradient coilcontrol unit 125 in communication with data processing system 105 may beused to control the operation of gradient coils 120.

In some implementations of MRI system 100, there may be additionalelectromagnet coils present (not shown), such as shim coils(traditionally, but not limited to, producing magnetic field profiles of2nd order or higher spherical harmonics) and/or a uniform field offsetcoil and/or any other corrective electromagnet. To perform activeshimming (correcting the field distortions that are introduced whendifferent objects are placed within or around the system), thecorrective electromagnets, such as the shim coils, carry a current thatis used to provide magnetic fields that act to make the main magneticfield more uniform. For example, the fields produced by these coils mayaid in the correction of inhomogeneities in the main magnetic field dueto imperfections in the main magnet 110, the presence of externalferromagnetic objects, or susceptibility differences of materials withinthe imaging region, or due to any other static or time-varyingphenomena.

The MRI system 100 further includes radio frequency (RF) coils 130. TheRF coils 130 are used to establish an RF magnetic field with strength B1to excite the atomic nuclei or “spins”. The RF coils 130 can also detectsignals emitted from the “relaxing” spins within the object beingimaged. Accordingly, the RF coils 130 may be in the form of separatetransmit coils 132 and receive coils 134 or a combined transmit andreceive coil (not shown) with a switching mechanism for switchingbetween transmit and receive modes.

The RF coils 130 may be implemented as surface coils, which aretypically receive-only coils and/or volume coils which can be receiveand transmit coils. RF coils 130 can be integrated in the main fieldmagnet 110 bore. Alternatively, RF coils 130 may be implemented incloser proximity to the object to be scanned, such as a head, and cantake a shape that approximates the shape of the object, such as aclose-fitting helmet. An RF coil control unit 135 in communication withdata processing system 105 may be used to control the operation of theRF coils 130 in either a transmit aspect or a receive aspect.

As shown in FIG. 1, data processing system 105 further includes a pulsesequence subsystem 160 for establishing one or more multiband pulsepairs for acquiring imaging data.

To obtain images from the MRI system 100, one or more sets of RF pulsesand gradient waveforms (collectively called “pulse sequences”) may begenerated at the pulse sequence subsystem 160.

In the described embodiment, pulse sequence subsystem 160 is configuredto initiate a plurality of multiband pulse pairs for simultaneouslyexciting or refocusing multiple bands. Each of the multiband pulse pairsincludes a multiband excitation pulse, which is configured tosimultaneously excite multiple bands, and a corresponding multibandrefocusing pulse, which is configured to simultaneously refocus themultiple bands.

Pulse sequence subsystem 160 constructs these multiband pulses so thatthe phases of the bands excited and refocused by the multiband pulseswithin one pulse pair are consistent, or the generally the same. Toachieve this, pulse sequence subsystem 160 may establish a multibandexcitation pulse and a train of the corresponding multiband refocusingpulse, both of which correspond to the same row of an orthogonalencoding matrix, as discussed further below. In some examples, theorthogonal encoding matrix may be a Hadamard encoding matrix. Otherorthogonal encoding matrices may be used as appropriate.

In order to enable separation of the multiple bands from the sampledsignal, and reconstruction of images from the individual bands, pulsesequence subsystem 160 is configured to initiate more than one, i.e. aplurality of, multiband pulse pairs wherein least two pulse pairs havefrequency bands with respective different phases. To achieve this, pulsesequence subsystem 160 transmits another multiband excitation pulse andcorresponding multiband refocusing pulse, for exciting and refocusingthe same set of multiple bands, using a different row of the orthogonalencoding matrix.

The number of multiband pulse pairs established by pulse sequencesubsystem 160 may correspond to, or be less than, the number of slicesin the multiple slices to be simultaneously imaged. For example, if fourslices are to be simultaneously imaged, pulse sequence subsystem 160 mayinitiate four multiband pulse pairs. Establishing more pulse pairs thanthe number of slices in the multiple slices may provide for redundancy.

In such a case, a fourth-order Hadamard encoding matrix having four rowscould be used and pulse sequence subsystem 160 may be configured to useeach row of the Hadamard encoding matrix to construct the four multibandpulse pairs.

Alternatively, pulse sequence subsystem 160 may be configured to usefewer than all of the rows of the Hadamard encoding matrix whenconstructing the plurality of multiband pulse pairs.

Data processing system 105 then communicates these pulse pairs to the RFcontrol unit 135 which instructs transmit coil 132 to generate each ofthe specified multiband pulse pairs in a multiband pulse sequence thatachieves spin-echo or multi-spin-echo imaging. For example, onemultiband pulse sequence generated by transmit coil 12 may include themultiband excitation pulse and a train of multiband refocusing pulsesfrom one of the multiband pulse pairs.

In other embodiments, data processing system 105 may communicate themultiband pulse pair information to the RF control unit 135 and thegradient control unit 125, which collectively generate the associatedwaveforms and timings for providing a sequence of pulses to perform ascan. As such, while transmit coil 132 is shown in FIG. 1 to be a partof RF coils 130, transmit coil 132 may also be a part of gradient coils120, such that the gradient coils 120 and the RF coils 130 collectivelygenerate multiband pulse sequences.

Upon receiving sampled signal data from receive coil 134, pulse sequencesubsystem 160 is further configured to multiply the signal data by theinverse of the orthogonal encoding matrix to separate the signals foreach of the multiple bands to reconstruct images at each of the multiplebands.

While pulse sequence subsystem 160 can be part of any type of MRIsystem, in some examples MRI system 100 may be a low-field, e.g.,approximately 0.5 T, MRI system.

Pulse sequence subsystem 160 also may not necessarily be a separatecomponent in data processing system 105, in that the general processorof data processing system 105 may perform some or all of the above-notedsteps in conjunction with the general memory of data processing system105. Functions of pulse sequence subsystem 160 will be discussed infurther detail below.

Referring now to FIG. 2, an embodiment of one multiband pulse sequenceis shown that may be established by pulse sequence subsystem 160 andgenerated by transmit coil 132. The example pulse sequence shown in FIG.2 includes a 90° multiband excitation pulse and a train of 180°multiband refocusing pulses. Each 90° multiband excitation pulse andeach 180° multiband refocusing pulse are configured to simultaneouslyrefocus multiple bands. Data is typically sampled/acquired followingeach refocusing pulse, as indicated by “daq” in FIG. 2

These multiband excitation and refocusing pulses have been constructedusing a row of an orthogonal encoding matrix such that the phases of the90° multiband excitation pulse and 180° multiband refocusing pulses areconsistent or generally the same. This results in the example pulsesequence of FIG. 2. As described in further detail below, to encode themultiple bands, for example, N bands, the entire pulse sequence isrepeated N times, with the multiband pulses corresponding to each of theN rows of the encoding matrix. In some examples, the encoding matrix mayhave more than N rows, in which case the multiband pulses may be derivedfrom a subset of the rows of the encoding matrix.

Referring now to FIG. 3, there is shown an example imaging method 300 inaccordance with an embodiment of the present disclosure. In someexamples, method 300 may be at least in part operated using the MRIsystem 100 as shown in FIG. 1. Additionally, the following discussion ofmethod 300 leads to further understanding of system 100. However, it isto be understood that system 100, and method 300 can be varied, and neednot work exactly as discussed herein in conjunction with each other, andthat such variations are within scope of the appended claims.

To generate a multiband pulse sequence based on specified constraints, aset of design parameters are obtained (e.g., via user input) at 302which correspond to the desired characteristics for a single frequencyband of the multiband pulse. Such parameters include, but are notlimited to, bandwidth, transition width, and ripple amplitude for thesingle band. Input may also include selection of an encoding matrixand/or the number of bands to be simultaneously excited. The relativespacing of the bands relative to one another within the multiband pulsemay also be specified. In the present embodiment, the encoding matrix isa Hadamard encoding matrix; other suitable encoding matrices may beused. The number of bands to be simultaneously excited may be calculatedusing a preferred or optimal number of bands for the particular imagingprotocol. For example, for maximum SNR, the number of bands is given bythe total number of slices desired in the imaging protocol, which may bebased on the anatomy of interest, divided by the number of excitationand refocusing-train intervals that can be interleaved within a singlerepetition time (TR) (the length of the TR will depend on the desiredcontrast and MRI system used). For the present discussion, generally Nbands will be simultaneously excited.

The duration of the imaging pass and the number of samples to beobtained from the RF pulses may also be determined (e.g., via userselection). An example protocol may use 16 to 24 refocusing pulses inthe train of refocusing pulses, with the signal data being sampled oracquired after each refocusing pulse, as referenced by “daq” in FIG. 2.

After the design parameters are set, then the pulse pairs are initiated.A least-square digital filter design is used at 304 to compute thefinite impulse response (FIR) filter coefficients corresponding to thesingle band filter design parameters, which correspond to a single bandRF signal. The FIR filter may be referred to as the “beta-polynomial”,B_(n)(z).

For each row in the selected encoding matrix (e.g., the Hadamard matrixin the present case), a modulation vector (M) is computed. Since N bandswere selected to be simultaneously excited, an N-th order Hadamardencoding matrix may be used to construct the pulses. As such, assumingthat the selected encoding matrix has N rows the modulation vector (M)for row i is computed at 306. The calculation of M may be iteratedthrough the necessary number of rows, from i=1 to i=N, until the desirednumber of pulse pairs has been obtained.

The single-band filter coefficients of B_(n)(z) are then multiplied at308 by the modulation function (M) for row i.

To establish a multiband excitation pulse, where each multibandexcitation pulse is configured to simultaneously excite multiple bands,the modulated filter is then scaled for a flip angle of 90 degrees at310. This scaled filter results in the beta-polynomial, which is one ofthe inputs of the Shinnar-Le Roux (SLR) transform. The beta polynomialsfor each band is then modulated to shift the band to the desired bandposition, and the sets of coefficients for each band are summed toproduce a composite Bn(z), so a multiband RF pulse may be retrieved.Using these summed beta-polynomials within the SLR transform enables thecontrol of the phases between the multiple bands when they are used forimaging.

The minimum phase alpha polynomial is then derived from the compositebeta-polynomial at 312.

The inverse SLR transform is then used on the computed alpha andbeta-polynomial at 314 to compute the 90° excitation RF pulse, i.e. themultiband excitation pulse.

To establish a refocusing multiband refocusing pulse, where eachrefocusing multiband pulse is configured to simultaneously refocus themultiple bands, the modulated filter noted above is then scaled for aflip angle of 180 degrees at 316, resulting in another multibandbeta-polynomial.

Another minimum phase alpha-polynomial is then derived from thiscomposite beta-polynomial at 318.

The inverse SLR transform is then used again on these subsequentlycomputed alpha and beta-polynomials at 320 to compute the 180°refocusing RF pulse, i.e. resulting in the multiband refocusing pulse.

In this manner, the multiband excitation pulse and the multibandrefocusing pulse corresponding to row i are established and stored at322. It should be understood that obtaining the multiband excitationpulse and obtaining the multiband refocusing pulse may be performed inparallel. It is then determined if a sufficient number of multibandpulse pairs has been obtained. For example, i may be incremented by oneand if i is not greater than N, then 306 to 322 are repeated for asubsequent row in the encoding matrix. In this manner, 306 to 322 wouldbe repeated N times to produce N pairs of a multiband excitation pulseand a corresponding multiband refocusing pulse that are suitable forspin-echo or multi-spin-echo multiband imaging.

These N multiband pulse pairs, each comprising a multiband 90°excitation pulse and a multiband 180° refocusing pulse, may then bestored for future application, or they may be generated and appliedduring multi-spin-echo imaging at 324 in multiband pulse sequences.

Initial generation of the N pulse pairs during multi-spin-echo imagingmay involve phase correction that is conventionally applied to the 90degree excitation pulse to correct for spurious phases that perturbs theCPMG condition. This phase correction is applied to the phase of thebands of the 90 degree multiband pulse. The phase correctionparameter(s) is(are) measured at each of the slice locations using aconventional single-band calibration scan at each of the N slicelocations. For example, the phase correction method taught in U.S. Pat.No. 5,378,985 could be used to perform the single-band calibration scan.For each slice location, the single-band calibration scan givesmeasurements of zeroth order phase corrections, as well a first-orderphase correction parameters for both the readout and phase-encodingdirections.

Substantially, due to the use of the same row of the encoding matrix toderive the phase of the bands in both the 90° and 180° pulse profiles,the zeroth order phase correction parameter measured using thesingle-band calibration scan for each band location can be applied tothe corresponding band of each of the N 90° multiband excitation pulses.Given that each excitation pulse and refocusing pulse are, respectively,90° and 180° pulses, a local Carr Purcell Meiboom Gill (CPMG) conditionis established in all of the simultaneously excited bands.

In one preferred embodiment of the invention, the bands of the multibandpulses are adjacent, as shown in FIG. 4, so that the first order phasecorrection parameters are similar for all of the bands excited by eachmultiband pulse. In this embodiment, the average of the first-ordercorrection parameters for the particular band locations can be used tocorrect the readout and phase-encode gradients used to image these bandlocations. In another preferred embodiment, the first-order phasecorrection parameters used to correct for both the readout andphase-encoding directions are not expected to vary with band (slice)location so these corrections can be applied to each of the N multibandacquisitions in the same manner as a conventional single-bandacquisition.

Signal data may be sampled/acquired (e.g., using the receive coil)during the application of each multiband pulse sequence, e.g., followingeach of the multiband refocusing pulses. The use of a train of multiplerefocusing pulses is typically used to get many phase encoding stepswithin a single repetition of the sequence. Each set of samples acquiredbetween refocusing pulses may form a column in a matrix of sampledsignal data. The signal data matrix may then be multiplied by theinverse of the encoding matrix to separate the signals for each of theindividual bands to reconstruct images at each of the individual bands.

In the present embodiment, since N different multiband pulse sequencesare applied to encode N slices, signal data from each individual band issampled a factor of N times more than in the corresponding single-bandpulse sequence. Thus, the net data acquisition time for each slice isincreased by a factor of N. As a result, signal from each slice issubjected to analog-to-digital conversion N times longer than would becase for a single-band acquisition.

As noted above, in order to improve the SNR, either the voxel size hasto increase or the sampling time has to increase. Therefore, accordingto the SNR equation, where:

SNR=V*T ^(1/2) *R(B0,B1, . . . )*I _(seq)(T1,T2,TE,TR, . . . )

wherein V is the voxel volume, T is the total sampling time for eachvoxel, R is a factor characterizing the SNR of the hardware andprocessing chain including the main magnetic field, the receive coilsensitivity etc, and I_(seq) is a factor characterizing the signalintensity from the pulse sequence and the tissue, increasing T by afactor of N would increase SNR by a factor of N^(1/2), i.e. a factor ofthe square root of N.

In this manner, the SNR for images acquired using MRI systems which uselower magnetic fields, such as 0.5 T, may be improved without vastlyincreasing sampling time.

In some examples, fewer than all of the rows of the encoding matrix maybe used to construct the beta-polynomials. However, images from theindividual bands may still be reconstructed. Typically, datacorresponding to more than one row of the encoding matrix must beacquired in order to reconstruct images from the individual bands.

Where fewer than all the rows of the encoding matrix are used toconstruct the multiband pulses, fewer than N multiband pulse sequencesare applied to encode the N slices simultaneously. In such a case,signal data from each individual slice is sampled M times, where M<N.Then, signal from each slice is subjected to analog-to-digitalconversion for M times longer than would be case for a single-bandacquisition. In this way, T would still be increased by a factor of M,and SNR would still increase by a factor of (M)^(1/2).

To mitigate lower SNR, various examples disclosed herein act to increasethe net data sampling time for each slice acquired in a multi-spin-echopulse sequence, such that the image SNR is comparable to what would beachieved at higher magnetic fields with a conventional multi-spin-echosequence.

FIG. 4 is an example of four pairs of a multiband excitation pulse and amultiband refocusing pulse that may be applied during spin-echo ormulti-spin-echo imaging for imaging four slices simultaneously accordingto an example of the present disclosure. Each row shows a multibandpulse pair comprising a multiband (90°) excitation pulse and a multiband(180°) refocusing pulse. The phases of the bands for both the 90° and180° pulses in each row are based on the same row of an encoding matrix,which substantially enables a CPMG condition at each of the bandlocations. As such, the only difference between the pulses in each rowis their phases.

In the example of FIG. 4, each of the four bands will be excited thenrefocused multiple times by each row of pulses, resulting in thespin-echo profiles of the third column.

Following multiplication of the signal data by the inverse of theorthogonal encoding matrix, the signals for each of the multiple sliceis reconstructed as shown in the reconstructed slice profiles of thefourth column of FIG. 4.

In this case, since four multiband pulse sequences are applied to excitefour bands simultaneously, signal data from each individual slice issampled four times, compared to a single-band acquisition. Thus, the netdata acquisition time for each slice is increased by a factor four, andSNR should increase generally by a factor of 4^(1/2)=2.

In various examples disclosed herein, a method and system of obtainingMR images with higher SNR for each slice in MRI systems which use lowermagnetic fields is provided. By using multiband pulses, the acquisitiontime for each band is effectively increased, thus enabling a higher SNRover the same total acquisition time, compared to conventionalapproaches.

While some embodiments or aspects of the present disclosure may beimplemented in fully functioning computers and computer systems, otherembodiments or aspects may be capable of being distributed as acomputing product in a variety of forms and may be capable of beingapplied regardless of the particular type of machine or computerreadable media used to actually effect the distribution.

At least some aspects disclosed may be embodied, at least in part, insoftware. That is, some disclosed techniques and methods may be carriedout in a computer system or other data processing system in response toits processor, such as a microprocessor, executing sequences ofinstructions contained in a memory, such as ROM, volatile RAM,non-volatile memory, cache or a remote storage device.

A computer readable storage medium may be used to store software anddata which when executed by a data processing system causes the systemto perform various methods or techniques of the present disclosure. Theexecutable software and data may be stored in various places includingfor example ROM, volatile RAM, non-volatile memory and/or cache.Portions of this software and/or data may be stored in any one of thesestorage devices.

Examples of computer-readable storage media may include, but are notlimited to, recordable and non-recordable type media such as volatileand non-volatile memory devices, read only memory (ROM), random accessmemory (RAM), flash memory devices, floppy and other removable disks,magnetic disk storage media, optical storage media (e.g., compact discs(CDs), digital versatile disks (DVDs), etc.), among others. Theinstructions can be embodied in digital and analog communication linksfor electrical, optical, acoustical or other forms of propagatedsignals, such as carrier waves, infrared signals, digital signals, andthe like. The storage medium may be the internet cloud, or a computerreadable storage medium such as a disc.

Furthermore, at least some of the methods described herein may becapable of being distributed in a computer program product comprising acomputer readable medium that bears computer usable instructions forexecution by one or more processors, to perform aspects of the methodsdescribed. The medium may be provided in various forms such as, but notlimited to, one or more diskettes, compact disks, tapes, chips, USBkeys, external hard drives, wire-line transmissions, satellitetransmissions, internet transmissions or downloads, magnetic andelectronic storage media, digital and analog signals, and the like. Thecomputer useable instructions may also be in various forms, includingcompiled and non-compiled code.

At least some of the elements of the systems described herein may beimplemented by software, or a combination of software and hardware.Elements of the system that are implemented via software may be writtenin a high-level procedural language such as object oriented programmingor a scripting language. Accordingly, the program code may be written inC, C++, J++, or any other suitable programming language and may comprisemodules or classes, as is known to those skilled in object orientedprogramming. At least some of the elements of the system that areimplemented via software may be written in assembly language, machinelanguage or firmware as needed. In either case, the program code can bestored on storage media or on a computer readable medium that isreadable by a general or special purpose programmable computing devicehaving a processor, an operating system and the associated hardware andsoftware that is necessary to implement the functionality of at leastone of the embodiments described herein. The program code, when read bythe computing device, configures the computing device to operate in anew, specific and predefined manner in order to perform at least one ofthe methods described herein.

While the teachings described herein are in conjunction with variousembodiments for illustrative purposes, it is not intended that theteachings be limited to such embodiments. On the contrary, the teachingsdescribed and illustrated herein encompass various alternatives,modifications, and equivalents, without departing from the describedembodiments, the general scope of which is defined in the appendedclaims. Except to the extent necessary or inherent in the processesthemselves, no particular order to steps or stages of methods orprocesses described in this disclosure is intended or implied. In manycases the order of process steps may be varied without changing thepurpose, effect, or import of the methods described.

1. A magnetic resonance imaging (MRI) system configured to perform amultiband pulse sequence to acquire a magnetic resonance image atmultiple slices simultaneously, the MRI system comprising: a processorconfigured to transmit a multiband pulse pair, the pulse paircomprising: a multiband excitation pulse for simultaneously excitingmultiple bands; and a multiband refocusing pulse for simultaneouslyrefocusing the multiple bands; wherein the phases of each band in themultiband excitation pulse and the multiband refocusing pulse are setaccording to a row of an orthogonal encoding matrix; and a transmit coilcoupled to the processor for generating a multiband pulse sequencecomprising the multiband excitation pulse and at least one multibandrefocusing pulse during spin-echo or multi-spin-echo imaging.
 2. Thesystem of claim 1 wherein the multiband pulse sequence generated by thetransmit coil comprises a train of multiple multiband refocusing pulsesduring multi-spin-echo imaging.
 3. The system of claim 2, wherein theprocessor is configured to establish a plurality of multiband pulsepairs using each of the rows of the orthogonal encoding matrix to setthe phases of each band in each multiband pulse pair.
 4. The system ofclaim 2, wherein the processor is configured to establish a plurality ofmultiband pulse pairs using fewer than all of the rows of the orthogonalencoding matrix to set the phases of each band in each multiband pulsepair.
 5. The system of claim 3, further comprising a receive coil forsampling signal data during application of each multiband pulsesequence.
 6. The system of claim 5, wherein the processor is furtherconfigured to multiply the signal data by the inverse of the orthogonalencoding matrix to separate the signals for each of the multiple bandsto reconstruct images encoded by each of the multiple bands.
 7. Thesystem of claim 1, wherein the multiband excitation pulse is a 90°multiband excitation pulse and the multiband refocusing pulse is a 180°multiband refocusing pulse transmitted with a 90° phase shift relativeto the excitation pulse, such that a local Carr Purcell Meiboom Gill(CPMG) condition is created in each of the multiple bands during theecho train.
 8. The system of claim 1, wherein the system is a low-fieldMRI system.
 9. A method of obtaining a magnetic resonance image byperforming magnetic resonance imaging (MRI) at multiple slicessimultaneously, the method comprising: generating a multiband pulsesequence for spin-echo imaging, the pulse sequence comprising amultiband excitation pulse and at least one multiband refocusing pulse,wherein the multiband excitation pulse simultaneously excites multiplebands; wherein the at least one multiband refocusing pulsesimultaneously refocuses the multiple bands; and wherein the phases ofthe bands excited by the multiband excitation pulse and the phases ofthe bands refocused by the at least one multiband refocusing pulse areset according to a single row of an orthogonal encoding matrix, themultiband excitation pulse and the at least one multiband refocusingpulse collectively forming a multiband pulse pair.
 10. The method ofclaim 9, wherein the multiband pulse sequence comprises multiplemultiband refocusing pulses.
 11. The method of claim 10, wherein thephase of a given band excited by the multiband excitation pulse and thephase of the corresponding band refocused by the multiband refocusingpulses are generally the same, in order to create a CPMG condition inall of the simultaneously excited bands.
 12. The method of claim 11,wherein the orthogonal encoding matrix comprises a Hadamard encodingmatrix.
 13. The method of claim 12, further comprising generating aplurality of multiband pulse sequences, wherein the phases of the bandsexcited by each multiband excitation pulse and the phases of the bandsrefocused by each multiband refocusing pulse in each respective pulsepair are set according to a respective row of the orthogonal encodingmatrix.
 14. The method of claim 12, further comprising generating aplurality of multiband pulse sequences, wherein the phases of the bandsexcited by each multiband excitation pulse and the phases of the bandsrefocused by each multiband refocusing pulse in each respective pulsepair are set according to fewer than all of the rows of the orthogonalencoding matrix.
 15. The method of claim 13, further comprisingacquiring signal data during generation of each multiband pulsesequence, and multiplying the signal data by the inverse of theorthogonal encoding matrix to separate the signals for each of themultiple bands to reconstruct images at each of the multiple slices. 16.The method of claim 9, wherein the number of bands in the multiple bandsis calculated using a total number of slices of the magnetic resonanceimage divided by the number of bands that can be interleaved within arepetition time of the MRI system.
 17. The method of claim 9, whereinthe multiband excitation pulse is a 90° excitation pulse and the atleast one multiband refocusing pulse is a 180° refocusing pulse tocreate a local Carr Purcell Meiboom Gill (CPMG) condition at each of themultiple bands.
 18. The method of claim 9, wherein the phases of thebands excited by the multiband excitation pulse and the phases of thebands refocused by the at least one multiband refocusing pulse are setaccording to a single row of an orthogonal encoding matrix by summingbeta-polynomials within a Shinnar-Le Roux transform to enable control ofthe phases between the multiple bands and retrieve the multibandexcitation pulse and corresponding multiband refocusing pulse.
 19. Themethod of claim 18, wherein each beta-polynomial is designed using adigital filter design algorithm.
 20. The method of claim 9, furthercomprising a zeroth order phase correction to the phase of each band inthe multiband excitation pulse.
 21. The method of claim 20, wherein thezeroth order phase correction is determined at the location of each ofthe bands using a single-band calibration scan at the location of eachof the bands.
 22. The method of claim 9, wherein the multiband pulsepair is for use in a low-field MRI system.